The present invention relates to a control device of an internal combustion engine.
In an exhaust system of an internal combustion engine which is mounted on a vehicle, an exhaust purification unit for removing CO (carbon monoxide), HC (hydrocarbon), NOx (nitrogen oxide), and particulate matter (PM), which are contained in the exhaust of the internal combustion engine, is provided.
In a case where the internal combustion engine is a gasoline engine, a three-way catalyst is widely used as the exhaust purification unit.
The three-way catalyst is a catalyst that oxidizes CO to CO2, oxidizes HC to CO2 and H2O and reduces NOx to O2 (oxygen) and N2 (nitrogen).
Furthermore, in a case where the internal combustion engine is a diesel engine, as the exhaust purification unit, the following are used in combination. That is, there are an oxidation catalyst, a NOx storage catalyst, and a diesel particulate filter (DPF).
The oxidation catalyst oxidizes CO and HC to CO2 (carbon dioxide) and H2O (water).
The NOx storage catalyst discharges NOx to reduce it to N2 and CO2 and remove the same by temporarily trapping the NOx of the exhaust to make the exhaust a reduction atmosphere.
The diesel particulate filter collects and removes the particulate matters contained in the exhaust.
In the above-mentioned gasoline engine, in order to perform an activation processing which activates the three-way catalyst, in other words, in order to raise the temperature of the three-way catalyst to improve the reactivity as the catalyst, a control, which temporarily modulates the air-fuel ratio to supply a partial unburned gas (hereinafter, simply called an unburned gas) to the three-way catalyst, is performed.
As described above, compulsorily modulating the air-fuel ratio is called a compulsory modulation. In addition, temporarily and compulsorily modulating the air-fuel ratio to be rich (a rich air-fuel ratio) is called a rich spike.
More specifically, the unburned gas supplied by modulating the air-fuel ratio is oxidized (burned) with the three-way catalyst to raise the temperature of the three-way catalyst, thereby activating the three-way catalyst.
Furthermore, in the above-mentioned diesel engine, in order to perform the reproduction processing of the exhaust purification unit, that is, the NOx reduction processing and the self purge of the NOx storage catalyst and the reproduction processing of the diesel particulate filter, a control for compulsorily modulating the air-fuel ratio in the same manner as described above is performed.
More specifically, the NOx reduction processing (a rich purge) is performed by modulating the air-fuel ratio to supply the unburned gas (a reducing agent) to the NOx storage catalyst.
In addition, the temperature of the exhaust is raised by about 600° C. by modulating the air-fuel ratio to oxidize (burned) the unburned gas with the oxidation catalyst or the NOx storage catalyst. In addition, the particulate matters converged in the diesel particulate filter are burned by supplying the diesel particulate filter with the exhaust with the temperature raised, whereby the reproduction processing of the diesel particulate filter is performed.
As described above, when the air-fuel ratio is temporarily and compulsorily modulated and changes, a fluctuation (fluctuation of the revolution or fluctuation of the torque) of the internal combustion engine (hereinafter, called an engine) occurs.
However, when a vehicle is driven at a certain speed (a normal driving) and when an engine is in an idle state, a feed back control of a driving control parameter of the engine functions so that the revolutions of the engine are constantly maintained.
Specifically, in the case of the diesel engine, the feed-back control of the ignition timing or the throttle opening degree as the driving control parameter functions.
In the case of the gasoline engine, the feed back control of the fuel injection amount as the driving control parameter functions.
Thus, even when the air-fuel ratio temporarily increases, a fluctuation in revolution of the engine and a fluctuation in torque are suppressed.
On the other hand, when the vehicle is accelerated, since the driving control parameter of the engine is subjected to a feed-forward control corresponding to an accelerator operation by a driver, the feed-back control does not function.
For this reason, for example, when the air-fuel ratio is temporarily modulated to be rich at the time of acceleration, it is difficult to avoid the generation of the fluctuation in the engine.
Furthermore, there is proposed a technique which suppresses the fluctuation in torque due to a temporary richness of the air-fuel ratio by delaying the ignition timing of the engine (see JP-A-2002-364414).
However, the above-mentioned technique can be applied to cases where the vehicle is normally driven or the engine is in the idle state, but it cannot be applied at the time of acceleration.
That is, when the feed-back control of the driving control parameter such as at the time of acceleration of the vehicle does not function and only the feed-forward control is performed, performing the compulsory modulation of the air-fuel ratio at the time of acceleration generates the fluctuation in engine and becomes a cause of deteriorated drivability.
As a result, the compulsory modulation of the air-fuel ratio is performed only when the drivability is given priority and the feed-back control of the driving control parameter functions, namely, when the vehicle is in the idle state.
However, when a situation where the compulsory modulation of the air-fuel ratio can be performed is restricted, it is disadvantageous in securing a processing that should be performed with respect to the exhaust purification unit, i.e., an implementation frequency of the activation processing of the exhaust purification unit.